The inventive subject matter disclosed herein generally relates to foot assessment systems to assess displacement of the talus relative to an axis of the foot and lower leg. More particularly, the inventive subject matter relates to a system and a method to assess the amount of pronation of a foot and lower extremity.
Pronation is a complex motion in three dimensions and involves rotation among multiple axes of the foot, ankle, and leg. Normally, the foot adapts to the surface it lands upon, slightly rotates inward, and flattens the arch as the heel hits the ground. The foot's ability to pronate assists the body's mid-stance balance and is a part of a natural shock-absorbing system.
Pronation is a tri-planar motion of the foot: it consists of eversion, abduction, and dorsiflexion at the subtalar joint. Eversion (and oppositely, inversion) occurs in the frontal plane: the foot everts when it twists outward and upward, rotating the plantar surface (or sole) away from the center. Abduction (and oppositely, adduction) occurs in the transverse plane: the foot abducts when it rotates laterally, away from center. Dorsiflexion (and oppositely, plantarflexion) occurs in the sagittal plane: the foot dorsiflexes when it moves upwards, toward the tibia.
Oppositely to pronation, supination is an outward rotation of the ankle while the outside border of the foot supports the body.
A normal amount of pronation and supination is beneficial; it is the body's way to absorb shock, create a more stable and rigid platform for push-off, and achieve dynamic balance. Excessive motion in either direction can be very problematic if not controlled and predisposes the lower extremity to injury. An excessive amount of pronation (over-pronation) can be problematic because the shifting causes increased stress on the inside, or medial aspect, of the foot, ankle, and lower leg. Over-pronation pulls on the stabilizing muscles in the lower leg (posterior tibialus). The body may compensate for over-pronation by excessive internal rotation of the lower extremity and shifting of the subtalar-joint axis and midtarsal joint axis medially, for example. This may result in injuries to the knee, ankle, lower leg, and Achilles tendon. Conversely, excessive supination stretches the stabilizing muscles on the outside of the lower leg (peroneals) and the ankle may roll over.
It is often necessary to correct foot-motion when a foot over pronates. The correction may greatly reduce the propensity for injury and improves ambulatory performance. An over-pronating foot can be corrected with remedial foot support that aligns the foot in a normal pronation-range. Orthotic-insoles, also called prescription foot orthotics, are one approach to properly align and support the foot. Orthotic insoles are custom-made inserts for shoes and are designed to correct various foot and lower body conditions. The manufacturing and materials vary based on patient needs, activities, and health factors. Another approach is to select motion-control shoes, or shoes with stabilizing features, that can correct the range of motion of an over-pronating foot.
To select the appropriate orthotic insole or motion-control shoe, the amount of pronation must be assessed. One prior-art assessment technique teaches videotaping the foot while running. Pronation is determined by viewing the motion of the rear foot, or movement of the calcaneus in the frontal plane. However, visually assessing pronation based on rear-foot motion is inaccurate. For example, a foot can exhibit a small amount of calcaneal eversion yet have severe over-pronation. Alternatively, the amount of calcaneal eversion can be limited in the frontal plane but severe rotation may occur at the metatarsal joint in the transverse plane.
Another prior art attempt to assess pronation requires a visual inspection of the worn tread on a pair of shoes. This is highly subjective, as it depends on the skill of the observer.
Another prior art assessment method quantifies the lowering of the longitudinal-arch profile. This method is unsatisfactory. For example, a foot can maintain a high arch-profile but exhibit severe transverse-plane and frontal-plane motion at the subtalar joint and midtarsal joint.
U.S. Pat. No. 4,662,079 discloses a process and apparatus for forming customized footwear by determining a range of motion from supination to pronation. The patent teaches that this is done while keeping the rear foot bone completely in a neutral position. A range-of-motion measuring apparatus is used to accurately determine the neutral position of the bone structure of the rear foot complex. However, the range of motion is determined while the foot is only semi-weight bearing, that is, when a person is sitting. The patent further discloses the use of pressure switches under a subject's foot to determine end points of motion. The prior art teaches that the neutral position of the rear foot is achieved when it is placed one third of the way from maximum pronation toward maximum supination. This has been shown to be inaccurate.
Each of these prior art methods does not efficiently or accurately assess pronation. The prior art systems are cumbersome, complicated, and require skill to use. The prior art systems are also imprecise and subjective. Therefore, there is a need for a more accurate and simpler approach of assessing pronation.